This invention relates to a portable machine tool, and particularly a machine tool for machining a large area.
Numerous machine tools have been designed to allow machining of a large area. For example, Appleton, U.S. Pat. No. 3,650,178, discloses a milling machine having two horizontal guide rails supporting a beam that is displaceable along the rails, and a machining head mounted on the beam and movable therealong. Klein, U.S. Pat. No. 3,523,485, discloses a machine that is similar in some ways to that of Appleton.
In certain applications, it is desirable to be able to mill a large area of work without transporting the work to a machine shop. The co-pending U.S. patent application Ser. No. 07/590,240 discloses a portable milling machine, which allows work to be machined in situ. The milling machines shown by Appleton and Klein are not designed to be portable and therefore are not suitable for this application.
A machine tool based on the design shown by Appleton is somewhat limited with respect to machining accuracy when the horizontal guide rails are spaced widely apart, because the beam is then deflected under its own weight between its points of support on the rails. Deflection can be prevented by employing a beam whose cross sectional shape is such that the beam is very stiff. Generally, however, a beam that is very stiff will have a large mass, and this is not favorable with regard to portability.
The portable milling machine disclosed in the co-pending U.S. patent application Ser. No. 07/590,240 is designed to allow milling of a surface that is disposed either above or below the milling tool. It is sometimes necessary also to be able to mill a surface that is disposed vertically, and in the case of a milling machine in which the machining head is mounted on a beam, this may necessitate rotation of the beam about its longitudinal axis through 90.degree. relative to its orientation when used to mill a horizontal surface. A beam whose cross-sectional shape is such that the beam is very stiff with respect to bending in a first plane is not generally stiff with respect to bending in a plane perpendicular to the first plane unless the beam has a large mass, and this is not favorable with regard to portability.